SSH key

Using keys has some major advantages over using SSH without keys, such as no more having to type in long and strong passwords all the time.

Public key cryptography uses a public key to encrypt data and a private key to decrypt it.
Public key: the contents of the ~/.ssh/id_rsa.pub file
For anyone to actually gain access they need the corresponding private key (i.e. the decrypted contents of ~/.ssh/id_rsa) to identify themselves.